1. Field of the Invention
This invention relates generally to packerfeet for use with downhole drilling tools, and specifically to a packerfoot having a bladder assembly configured for longer life and reduced risk of delamination of the bladder.
2. Description of the Related Art
Tools for moving within underground boreholes are used for a variety of purposes, such as oil drilling, mining, laying communication lines, and many other purposes. In the petroleum industry, for example, a typical oil well comprises a vertical borehole that is drilled by a rotary drill bit attached to the end of a drill string. The drill string may be constructed of a series of connected links of drill pipe that extend between ground surface equipment and the aft end of the downhole tool. Alternatively, the drill string may comprise flexible tubing or xe2x80x9ccoiled tubingxe2x80x9d connected to the aft end of the tool. A drilling fluid, such as drilling mud, is pumped from the ground surface equipment through an interior flow channel of the drill string and through the tool to the drill bit. The drilling fluid is used to cool and lubricate the bit, and to remove debris and rock chips from the borehole, which are created by the drilling process. The drilling fluid returns to the surface, carrying the cuttings and debris, through the annular space between the outer surface of the drill pipe and the inner surface of the borehole.
Tools for moving within downhole passages are often required to operate in harsh environments and limited space. For example, tools used for oil drilling may encounter hydrostatic pressures as high as 16,000 psi and temperatures as high as 300xc2x0 F. Typical boreholes for oil drilling are 3.5-27.5 inches in diameter. Further, to permit turning, the tool length should be limited. Also, downhole tools must often have the capability to generate and exert substantial force against a formation. For example, operations such as drilling require thrust forces as high as 30,000 pounds.
As a result of the harsh working environment, space constraints, and force generation requirements, downhole tractors are used only in very limited situations, such as within existing well bore casing and smaller diameter open holes. While a number of the inventors of this application have previously developed a significantly improved design for a downhole tractor, further improvements are desirable to achieve performance levels that would permit downhole tractors to achieve commercial success in other environments, such as open bore drilling.
In one known design, a tool for moving within an underground passage comprises an elongated body, a propulsion system for applying thrust to the body, and packerfeet for anchoring the tractor to the inner surface of a borehole or passage while such thrust is applied to the body. Each packerfoot has a bladder having an inflated position in which the bladder grips the inner surface of the passage to substantially prevent relative movement therebetween, and a deflated position in which the bladder permits substantially free relative movement between the bladder and the inner surface of the passage. Typically, each packerfoot is longitudinally slidable with respect to the tool body so that the body can be thrust longitudinally while the packerfoot""s bladder is inflated. The exterior surfaces of the bladders are preferably configured so as not to substantially impede xe2x80x9cflow-by,xe2x80x9d the flow of fluid returning from the drill bit up to the ground surface through the annulus between the tool and the borehole surface.
Tools for moving within downhole passages may have at least two packerfeet that can be alternately inflated and deflated to assist the motion of the tool. As used herein, inflating or deflating a packerfoot refers to the inflation or deflation of the bladder of the packerfoot. In one cycle of operation, the body is thrust longitudinally along a first stroke length while a first packerfoot is inflated and a second packerfoot is deflated. During the first stroke length, the second packerfoot moves along the tool body in a reset motion. Then, the second packerfoot is inflated and the first packerfoot is subsequently deflated. The body is thrust longitudinally along a second stroke length. During the second stroke length, the first packerfoot moves along the tool body in a reset motion. The first packerfoot is then inflated and the second packerfoot subsequently deflated. The cycle then repeats. Alternatively, a tool may be equipped with only a single packerfoot for specialized applications of well intervention, such as movement of sliding sleeves or perforation equipment.
Packerfeet are typically powered by fluid, such as drilling mud in an open system or hydraulic fluid in a closed system. Typically, pressurized fluid is delivered to the bladder interior to inflate the bladder. When it is desired to deflate the bladder, the bladder interior is brought into fluid communication with the annulus between the tool and the inner surface of the passage to dispel the fluid to the annulus. Motor-operated or hydraulically controlled valves in the tool body can control the delivery of fluid to and from the bladders.
The inventors of the present application have developed several designs of packerfeet incorporating bladders. One such design is described in U.S. Pat. No. 6,347,674, entitled xe2x80x9cELECTRICALLY SEQUENCED TRACTOR.xe2x80x9d In this design, the bladder comprises several generally tubular layers of rubber reinforced by fibers, such as fiberglass. The tubular layers are equal in length and arranged concentrically so that the ends of the layers are aligned with one another. Radial exterior surfaces of end portions of the bladder are bonded to rigid tubular members of the downhole tool. Additionally, a plurality of elongated flexible beams or xe2x80x9ctoesxe2x80x9d are bonded to the radial exterior surface of the bladder, for improved torsional rigidity. The toes have a trapezoidal cross-section to permit increased flow-by of drilling fluid through the annulus between the tool and the inner surface of the passage. The ends of the toes are retained against the bladder by two retaining rings, one at each end of the bladder.
It is an object of the present invention to provide a packerfoot having a bladder assembly configured for longer life and reduced risk of delamination of the bladder from the remainder of the packerfoot.
The present invention provides a packerfoot with a generally tubular bladder having first and second ends each having a plurality of annular steps that mate with and are bonded to annular steps of annuli or bladder attachment portions of the packerfoot. This configuration results in more even distribution of loads from the bladder to the bladder attachment portions, increasing the life of the bladder.
A further aspect of the present invention comprises compressible strain-relief rings utilized in conjunction with a packerfoot having toes retained against a bladder by retaining rings. The strain-relief rings are positioned between the toe ends and the retaining rings. The strain-relief rings permit a degree of outward radial displacement of the toe ends during bladder inflation, thus limiting stress concentrations in the bladder and increasing bladder life.
A still further aspect of the present invention comprises a packerfoot in which the bond length of each end of the bladder to a rigid portion of the packerfoot is chosen so that the ratio of such bond length to the bladder thickness is greater than 25, and more preferably greater than 50. It has been discovered that bond lengths of this magnitude result in unexpectedly high increases in joint efficiency of the bladder to the rigid portions of the packerfoot.
A still further aspect of the present invention comprises a packerfoot in which the total bond length of the bladder to the rigid portions of the packerfoot is greater than or equal to twice the length of the inflatable portion of the bladder. Such proportionality between the total bond length and the length of the inflatable portion has been found to achieve unexpectedly improved resistance to failure. In another aspect, each of the first and second bond lengths is greater than or equal to the length of the inflatable portion of the bladder.
In accordance with one aspect, the present invention provides a gripper for anchoring a tool within, a passage, comprising an elongated mandrel, first and second elongated annuli, and an elongated generally tubular inflatable bladder. The mandrel is adapted to longitudinally slidably engage an elongated body of a tool. The first annulus is engaged with and surrounds a length of the mandrel. An end of the first annulus has a surface facing radially inward and having a plurality of annular steps. Similarly, the second annulus is engaged with and surrounds a length of the mandrel. An end of the second annulus has a surface facing radially inward and having a plurality of annular steps. The bladder has a retracted position in which the gripper permits substantially free relative movement between the gripper and an interior surface of the passage, and an inflated position in which the gripper substantially limits relative movement between the gripper and the interior surface of the passage. A first end of the bladder has a surface facing radially outward and having a plurality of annular steps. A second end of the bladder has a surface facing radially outward and having a plurality of annular steps. The annular steps of the first end of the bladder mate with and are adhered to the annular steps of the first annulus. Similarly, the annular steps of the second end of the bladder mate with and are adhered to the annular steps of the second annulus.
In accordance with another aspect, the present invention provides a gripper for anchoring a tool within a passage, comprising an elongated mandrel, first and second elongated bladder attachment portions or annuli, an elongated generally tubular inflatable bladder, a plurality of flexible beams on a radial exterior of the bladder, a retaining ring, and a strain-relief ring. The mandrel is adapted to longitudinally slidably engage an elongated body of a tool. The first and second bladder attachment portions are slidable with respect to the mandrel. The bladder has a first end bonded to the first bladder attachment portion and a second end bonded to the second bladder attachment portion. The bladder has a retracted position in which the gripper permits substantially free relative movement between the gripper and an interior surface of a passage, and an inflated position in which the gripper substantially limits relative movement between the gripper and the interior surface of the passage.
The beams are oriented generally parallel to the mandrel and have ends positioned radially exterior of the bladder. The retaining ring is positioned radially exterior of the ends of the beams and substantially prevents radial outward movement of such ends of the beams. The strain-relief ring is wedged between the retaining ring and the ends of the beams. Central regions of the beams are configured to flex radially outward to grip a passage surface when the bladder is in the actuated position. The strain-relief ring is configured to compressibly permit a degree of radial outward movement of the ends of the beams as the central regions flex radially outward. In one embodiment, an additional retaining ring and strain-relief ring pair is positioned on the opposite ends of the toes.
In another aspect, the present invention provides a packerfoot for anchoring a tool within a borehole, comprising an elongated mandrel, first and second elongated bladder attachment portions slidable with respect to the mandrel, and an elongated generally tubular inflatable bladder. The mandrel is adapted to longitudinally slidably engage an elongated body of a tool. The bladder has a retracted position in which the bladder permits substantially free relative movement between the packerfoot and an inner surface of a borehole, and an inflated position in which the bladder substantially limits relative movement between the packerfoot and the inner surface of the borehole. The bladder has a first end and a second end. The first end is bonded to the first bladder attachment portion along a first bond length of the bladder. The second end is bonded to the second bladder attachment portion along a second bond length of the bladder. The bladder has an inflatable portion separate from its first and second bond lengths. The sum of the first and second bond lengths of the bladder is greater than or equal to twice the length of the inflatable portion of the bladder.
In yet another aspect, the present invention provides a packerfoot for anchoring a tool within a borehole, comprising an elongated mandrel, first and second elongated bladder attachment portions slidable with respect to the mandrel, and an elongated generally tubular inflatable bladder. The mandrel is adapted to longitudinally slidably engage an elongated body of a tool. The bladder has a retracted position in which the bladder permits substantially free relative movement between the packerfoot and an inner surface of a borehole, and an inflated position in which the bladder substantially limits relative movement between the packerfoot and the inner surface of the borehole. The bladder has a first end and a second end. The first end is bonded to the first bladder attachment portion along a first bond length of the bladder. The second end is bonded to the second bladder attachment portion along a second bond length of the bladder. The bladder has a fiber-reinforced component. Both (1) the ratio of the first bond length of the bladder to the thickness of the fiber-reinforced component and (2) the ratio of the second bond length of the bladder to the thickness of the fiber-reinforced component are greater than 25.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above and as further described below. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.